1. Field of the Invention
The invention relates to the manufacture of laminar electrodes having a selectively ion-permeable diaphragm and used for determining specific ion concentration (e.g. Na.sup.+, K.sup.+, Mg.sup.+, Ca.sup.+2, Cl.sup.-, etc.) in aqueous solutions such as a biological fluids.
2. Description
There are already a large number of prior-art electrodes and similar devices for selectively determining ions in aqueous solutions. The electrodes usually comprise an internal electrochemical reference half-cell fixed at a stable potential and adapted, in conjunction with an external reference electrode immersed simultaneously with the measuring electrode in a solution to be analyzed, to form an electrochemical cell of a potential dependent on the presence of a given ion in the solution. As described in the literature, a prior art assembly of this kind comprising two electrodes and a connecting electrolyte and it is described as a "battery". We shall avoid using this term, since a "battery" produces current in contrast to the present cell, which delivers a potential only in order to avoid irreversibility conditions.
The potential of the ion is proportional to the logarithm of the activity of the chosen ion in accordance with NERNST's well known equation E=E.sub.o .+-.KlnC.sub.i where E.sub.o is the standard potential, K is a constant and C.sub.i is the concentration of the ion. The potential is therefore proportional to the conentration of the ion, which can then be deduced by comparison with standard solutions.
For example, Swiss Patent No. 604,167 and U.S. Pat. Nos. 4,214,968; 4,053,381; 4,171,246 (EASTMAN KODAK) describe electrodes in the form of dry multi-laminates comprising, in the following order, an insulating support, a metal electrically conductive layer covered with an insoluble salt of the metal, a part of the layer being used as a terminal for connection to an electrometer, a reference electrolyte and an ion-selective diaphragm for selectively measuring a given ion. The assembly made up of the metal layer, the insoluble salt and the reference electrolyte constitutes the internal reference half-cell at stable potential. Alternatively the assembly can be replaced by a conductive layer covered with a redox system, e.g. the quinone-hydroquinone couple, which system likewise comprises an internal reference cell at stable potential.
The ionically selective diaphragm usually comprises a plasticizer and a hydrophobic matrix containing in dispersion an ionophoric substance, i.e. one used for selectively detecting a given ion to the exclusion of any other ions in the solution to be analyzed. Compare with W. E. MORF et al., Ion-Selective Electrodes in Analytical Chemistry, Vol. I, FRIESER Editor, Plenum Press (1981), pp 221 ff.
U.S. Pat. No. 3,856,649 (MILES) describes an electrode having a similar structure except that the conductive element is filiform instead of laminar and the electrolyte layer is appreciably hydrated.
U.K. Pat. No. 2,106,253 (FUJI) likewise describes an electrode for selectively determining the concentration of ions, the electrode being a laminate comprising an insulating support, a conductive layer covered with an insoluble salt of the metal forming the conductive layer, and a hydrophobic layer of an ion-selective material (ISM) covering the other elements. This document also describes a simplified variant electrode omitting not only the reference electrolyte but also the insoluble salt. In this variant the electrode comprises only the conductive layer covered with ISM material (see also Anal. Chem. 44 (1972), 856).
U.S. Pat. No. 4,454,007 (DUPONT) likewise describes a laminar ion-selective electrode having the following structure: a baseplate of insulating material, a layer of conductive material, a layer comprising powdered carbon dispersed in a dielectric polymer, and finally an ion-selective diaphragm made of material which penetrates with the material in the preceding layer at their junction plane.
In conventional practice, the previously-described electrodes are used as follows. When testing a solution for analysis, a drop thereof is deposited onto the diaphragm, which has selective ion permeability. The drop is also placed in contact with the external reference electrode, e.g. via a salt bridge, the terminals of the ion-selective electrode and the reference electrode being connected to a suitable electrometer for reading the potential. In numerous prior-art cases, the external reference electrode may be similar to or identical with the measuring electrode, and the external reference potential can be supplied either by a standard solution deposited simultaneously with the solution to be analyzed (in which case the solutions make contact by diffusion into a porous element situated between the deposition areas) or by a fixed reference element (the salt bridge for example) forming part of the external reference electrode.
In order to locate and fix the position of the drop of solution to be analyzed (and also the drop of standard solution when necessary) the ISM diaphragm is usually covered with a layer of insulating, waterproof material formed with compartments or windows giving access to only that portion of the ISM layer facing the window. This system prevents the drop spreading on the surface of the ISM layer and can also be used for selecting a fixed preset quantity of liquid for measuring, since the capacity of the compartments is kept constant from the electrode to the other.
Electrodes of this kind, however, are capable of producing short-circuits as a result either of inter-layer diffusion of aqueous liquids or if the liquids accidentally come into contact with the edges of the electrodes. Attempts have been made to obviate these defects by various means, e.g. by tightly sealing the edges of the electrodes (GA-A-2 106 253) or by grooving the conductive layer so as to divide it into zones and filling the grooves with ISM material (GA-A-2 121 183).